If you do a cam, valvetrain, ported & polished head and intake manifold, and headers, you'll probably keep up. All that will net you around 50hp, and will cost close to the cost of a b. Think about this, swap in the b, then do all that work, which will cost about 2-2.5k more. Now you have like 200hp to the wheels with a total cost of around 4k. Either way, get rid of that d-series.